The present invention relates to a radar system and method for detecting moving targets. More specifically, the present invention relates to a synthetic aperture radar (SAR) system for identifying moving targets from a set of data returns received at a single receive phase center.
Aircraft-borne radar systems have been designed in the past as ground mobile target indicators (GMTI). An airborne GMTI radar system radiates a plurality of electromagnetic pulses to illuminate a specific area on the ground. These pulses reflect off of moving targets as well as stationary clutter and targets. The reflected pulses are received by the radar system. The received pulses are processed by the radar system to reject stationary clutter and targets, and detect moving targets.
A conventional GMTI radar system utilizes multiple antennas to accomplish moving target detection. The number of antennas varies from 3 to 8. These antennas could be separate dish antennas, or different parts of a phased array antenna system. In the transmit mode, one of these antennas radiates from a transmit phase center. In alternative transmit modes, all or a selected group of the antennas radiate coherently together. In other words, they act as a single larger antenna with a single transmit phase center. The transmit phase center is the point from which the outward spreading electromagnetic pulses would seem to have originated, as seen by an observer from a distance. When the radar system radiates, there is a single transmit phase center, the location of which is controlled by electrical means and is not necessarily at the spatial center of the transmit antennas.
In the receive mode, the 3-8 antennas receive the reflected pulses as separate antennas. In other words, the antennas deliver separate outputs corresponding to different receive antenna phase centers. The separation of moving targets from stationary clutter is a key objective of a GMTI radar system. A variety of implementations have been demonstrated to meet this objective, and to meet a variety of other system requirements. Some of these implementations are referred to as Synthetic Aperture Radar (SAR) processing, Doppler Radar processing, Displaced Phase Center Antenna (DPCA) processing, and Space Time Adaptive Processing (STAP). A discussion of DPCA and SAR based moving target detection is given by Mehrdad Soumekh, xe2x80x9cSynthetic Aperture Radar Signal Processing with MATLAB Algorithms,xe2x80x9d John Wiley and Sons, Inc., New York, 1999, page 561 to 585.
Conventional implementations of a GMTI radar system require multiple separate receiving antennas. Described as follows are two conventional methodologies for performing GMTI. The first methodology for performing GMTI relies on achieving clutter rejection based on spatial correlation between received pulses. The second methodology for performing GMTI relies on achieving clutter rejection based on both spatial and temporal correlation of consecutive received pulses.
The first methodology for performing GMTI will be described with respect to a conventional GMTI radar system that is designed to synthesize a very large array antenna, using an aircraft that flies along a straight line. This is a basic configuration of synthetic aperture radar (SAR). The SAR radiates from different locations along the flight path of the aircraft. These locations are denoted by {s1, s2, s3, s4, . . . , sn, . . . }, which denotes a set of closely spaced points along a straight line. The motion of the aircraft causes the transmit phase center of the GMTI radar system to pass through these points. When the transmit phase center of the radar system is aligned with one of these points, the radar radiates a pulse. Since the aircraft flies at a constant velocity, the radar radiates at regular intervals.
The pulse repetition frequency (PRF) of the radar and the velocity of the aircraft are chosen so that the reflected pulses would be arriving at the radar in a proper manner, described as follows.
FIG. 1A depicts a GMTI radar system 100 that has 5 antennas with phase centers denoted by c0, c1, c2, c3, and c4. The radar system selects the phase center denoted by c3 as the transmit phase center and radiates a pulse from all 5 antennas. At the time of radar transmission the antenna phase centers are aligned with the locations {s98, s99, s100, s101, s102}. Thus, the radar effectively radiates a pulse from the location s101. A short interval later, the reflected pulse arrives.
FIG. 1B illustrates a short time later when the reflected pulse is received back by the radar. By the time the aircraft radar system receives the reflected pulse, the aircraft has moved forward, and the phase centers of the antennas, {c0, c1, c2, c3, c4}, are aligned with {s99, s100, s101, s102, s103}. The radar system uses these antennas to receive the reflected pulse. Each of the five antennas detects radiation, generating five receive xe2x80x9coutputsxe2x80x9d or xe2x80x9cchannelsxe2x80x9d. The output from the receiving antenna at c2 is denoted the mono-static SAR signal The terminology xe2x80x9cmono-staticxe2x80x9d refers to the notion that the location for the transmission of the out-going pulse (or subsequent out-going pulses) and the reception of the reflected pulse (or subsequent reflected pulses) are the same. The outputs from other receive channels are denoted bi-static SAR signals. The terminology xe2x80x9cbi-staticxe2x80x9d refers to the arrangement where the locations for transmit and receive are different. For example, the signal received by antenna C2 in FIG. 1B is a mono-static signal because it is received allocation S101 and the pulse was effectively radiated from location S101 (shown in FIG. 1A). The signals received by antennas C0, C1, C3, and C4 in FIG. 1B are bi-static signals.
The mono-static SAR signal is used to construct an image of the target area based on SAR processing. The resultant image is denoted a mono-static SAR image. The bi-static outputs are used to construct bi-static SAR images. If the clutter and targets in the area illuminated by the radar system are stationary, then it can be shown that any one of the bi-static SAR images could be used to produce an estimate of the mono-static SAR image, or vice versa.
Since multiple bi-static images are available, they can be combined to produce a good estimate of the mono-static image. The stationary clutter or targets that appear in the mono-static SAR image can be suppressed or substantially eliminated by subtracting the mono-static SAR image by an estimated version of the mono-static SAR image. The result of subtracting two images is another image, where the image features due to stationary clutter and targets are substantially diminished but the image features due to moving targets are enhanced.
For clarity of discussion, the above example ignored a practical matter. When the receive antennas {c0, c1, c2, c3, c4}, are aligned with {s99, s100, s101, s102, s103} as in FIG. 1B and are configured to receive, the above example implies that the GMTI radar system is configured to radiate a pulse at the same time, so that subsequently, another reflected pulse would arrive at a later time. In practice, a GMTI radar system would not radiate and receive at the same time, rather radar pulses should be radiated between intervals of reception.
In summary, a first conventional GMTI radar system and method is comprised of several receive antennas. Several SAR images are produced using the signals received by spatially separated antennas. At one of the intermediate data processing steps, one SAR image is produced using the data from a specific receive channel (the mono-static signal), and an estimated version of the SAR image is produced using the data from the other receive channels. Clutter rejection is achieved by taking the difference of the two images. This approach to clutter rejection is based on the correlation that exists between the outputs of spatially separated receive antennas.
A second approach to achieving clutter rejection is based on the temporal correlation that exists in consecutively received pulses. Both spatial and temporal correlation may be utilized in a combined algorithm as the following example illustrates.
FIGS. 2A-2C depict a GMTI radar system 200 that has 5 antennas with phase centers denoted by c0, c1, c2, c3, and c4. As shown in FIG. 2A, radar system 200 is programmed to radiate from the transmit phase center c2. As shown in FIG. 2B, when the antennas c0, c1, c2, c3, and c4 are aligned with the locations s98, s99, s100, s101, s102, a reflected pulse arrives and the radar system records the outputs from the antenna, and denotes the data x0(98), x1(99), x2(100), x3(101), and x4(102). The subscripts 0, 1, 2, denote the spatially separated receive channels. The indices 98, 99, 100, 101, 102 denote the time increments in time along slow-time dimension. Slow-time refers to the time dimension marked by a pulse repetition interval (PRI), which is the reciprocal of PRF. After one pulse repetition interval, the aircraft has traveled forward and the antennas are aligned with s99, s100, a101, s102, and s103, as shown in FIG. 2C. The radar receives another reflected pulse. The outputs are denoted x0(99), x1(100), x2(101), x3(102), and x4(103). A matrix of data is assembled, as illustrated by:                               x          0                ⁡                  (                      k            -            4                    )                                              x          1                ⁡                  (                      k            -            3                    )                                              x          2                ⁡                  (                      k            -            2                    )                                              x          3                ⁡                  (                      k            -            1                    )                                              x          4                ⁡                  (          k          )                                                  x          0                ⁡                  (                      k            -            3                    )                                              x          1                ⁡                  (                      k            -            2                    )                                              x          2                ⁡                  (                      k            -            1                    )                                              x          3                ⁡                  (          k          )                                              x          4                ⁡                  (                      k            +            1                    )                                                  x          0                ⁡                  (                      k            -            2                    )                                              x          1                ⁡                  (                      k            -            1                    )                                              x          2                ⁡                  (          k          )                                              x          3                ⁡                  (                      k            +            1                    )                                              x          4                ⁡                  (                      k            +            2                    )                                                  x          0                ⁡                  (                      k            -            1                    )                                              x          1                ⁡                  (          k          )                                              x          2                ⁡                  (                      k            +            1                    )                                              x          3                ⁡                  (                      k            +            2                    )                                              x          4                ⁡                  (                      k            +            3                    )                                ⋮              ⋮              ⋮              ⋮              ⋮                                    x          0                ⁡                  (          98          )                                              x          1                ⁡                  (          99          )                                              x          2                ⁡                  (          100          )                                              x          3                ⁡                  (          101          )                                              x          4                ⁡                  (          102          )                                                  x          0                ⁡                  (          99          )                                              x          1                ⁡                  (          100          )                                              x          2                ⁡                  (          101          )                                              x          3                ⁡                  (          102          )                                    xe2x80x83                                          x          4                ⁡                  (          103          )                                    xe2x80x83                            xe2x80x83                            xe2x80x83                            xe2x80x83                                          x          0                ⁡                  (          100          )                                              x          1                ⁡                  (          101          )                                              x          2                ⁡                  (          102          )                                              x          3                ⁡                  (          103          )                                    xe2x80x83                                          x          4                ⁡                  (          104          )                                    xe2x80x83                            xe2x80x83                            xe2x80x83                            xe2x80x83                        ⋮              ⋮              ⋮              ⋮              ⋮      
Each column in the above table is referred to as a channel of data. An SAR image can be created from any one of the channels of data. The five channels of data x0-x4 have a high degree of correlation among them. A space-time linear backward forward prediction data model may be constructed to predict x2(k) based on data from x0(k) and x1(k). The data model for predicting x2(k) based on x0(k) and x1(k)is:
a0 x0(kxe2x88x923)+a1 x1(kxe2x88x922)+a2 x0(kxe2x88x922)+a3 x1(kxe2x88x921)+a4 x0(kxe2x88x921)+a5 x1(k)=x2(k).
The set of coefficients {a0, a1, a2, a3, a4, a5} is a solution to the above equation for a single value of k. An error function, Ea, may be defined:             E      a        =                  ∑                              xe2x80x83                    ⁢          k                          xe2x80x83                    ⁢              xe2x80x83            ⁢                        "LeftBracketingBar"                                                                                                                a                      0                                        ⁢                                                                  x                        0                                            ⁡                                              (                                                  k                          -                          3                                                )                                                                              +                                                            a                      1                                        ⁢                                                                  x                        1                                            ⁡                                              (                                                  k                          -                          2                                                )                                                                              +                                                                                                                                                a                      2                                        ⁢                                                                  x                        0                                            ⁡                                              (                                                  k                          -                          2                                                )                                                                              +                                                            a                      3                                        ⁢                                                                  x                        1                                            ⁡                                              (                                                  k                          -                          1                                                )                                                                              +                                                                                                                                                a                      4                                        ⁢                                                                  x                        0                                            ⁡                                              (                                                  k                          -                          1                                                )                                                                              +                                                            a                      5                                        ⁢                                                                  x                        1                                            ⁡                                              (                        k                        )                                                                              -                                                            x                      2                                        ⁡                                          (                      k                      )                                                                                                    "RightBracketingBar"                2              ,      k    =    1    ,  2  ,  3  ,  ⋯  ⁢      xe2x80x83    ,      N    .  
The desired solution is that which minimizes the error function Ea for a range of k values. The solution {an} denotes the coefficients that may be used to predict the data x2 using the data x0 and x1.
A similar solution, denoted {bn}, may be developed to predict x2 using the data from the x3 and x4 channels. A data model is:
xe2x80x83b0x3(k)+b1x4(k+1)+b2x3(k+1)+b3x4(k+2)+b4x3(k+2)+b5x4(k+3)=x2(k),
and a desired solution {bn} may obtained by minimizing the error function, Eb,             E      b        =                  ∑                              xe2x80x83                    ⁢          k                          xe2x80x83                    ⁢              xe2x80x83            ⁢                        "LeftBracketingBar"                                                                                                                b                      0                                        ⁢                                                                  x                        3                                            ⁡                                              (                        k                        )                                                                              +                                                            b                      1                                        ⁢                                                                  x                        4                                            ⁡                                              (                                                  k                          +                          1                                                )                                                                              +                                                                                                                                                b                      2                                        ⁢                                                                  x                        3                                            ⁡                                              (                                                  k                          +                          1                                                )                                                                              +                                                            b                      3                                        ⁢                                                                  x                        4                                            ⁡                                              (                                                  k                          +                          2                                                )                                                                              +                                                                                                                                                b                      4                                        ⁢                                                                  x                        3                                            ⁡                                              (                                                  k                          +                          2                                                )                                                                              +                                                            b                      5                                        ⁢                                                                  x                        4                                            ⁡                                              (                                                  k                          +                          3                                                )                                                                              -                                                            x                      2                                        ⁡                                          (                      k                      )                                                                                                    "RightBracketingBar"                2              ,      k    =    1    ,  2  ,  3  ,  ⋯  ⁢      xe2x80x83    ,      N    .  
Using {an} and {bn}, two versions of the x2 channel may be predicted, as denoted by x2a and x2b. The results are three channels of data, x2a, x2 and x2b. Subsequently three SAR images corresponding to these channels may be constructed. Clutter rejection can be obtained by take the difference between two of images constructed using the data from these channels.
In summary, a second conventional GMTI/clutter suppression approach has been described. Two estimates of the xe2x80x9ccenterxe2x80x9d channel SAR signal, x2, are produced from a SAR with multiple receive antennas. The estimates, x2b and x2a may be referred to as xe2x80x9cearlyxe2x80x9d and xe2x80x9clatexe2x80x9d channels. The xe2x80x9ccenterxe2x80x9d channel SAR signal, x2, is used to produce a xe2x80x9ccenterxe2x80x9d channel SAR image. Subsequently, the xe2x80x9cearlyxe2x80x9d and xe2x80x9clatexe2x80x9d channels are used produced using two estimates of the xe2x80x9ccenterxe2x80x9d channel SAR image. These estimates are denoted xe2x80x9cearlyxe2x80x9d and xe2x80x9clatexe2x80x9d estimates of SAR images. Clutter rejection is obtained by taking the difference using two of the three SAR images or some linear combination of these images.
The two conventional GMTI methodologies just described must overcome several challenges. First, the aircraft does not fly along a desired flight path precisely as prescribed. Second, different antennas and receivers on a GMTI radar system do not have an identical response. The aircraft motion, and channel-to-channel mismatch must be overcome before a GMTI radar system can operate properly. The effects due to aircraft motion are corrected by one or more processes, collectively referred to as motion compensation. The effects due to channel-to-channel mismatch are corrected by one or more processes, collectively referred to as radar calibration.
The space-time backward forward linear prediction data model represents an approach to compensate for the effects of channel-to-channel mismatch. The subject of we motion compensation is covered in a number of text books. Two examples are Chris Oliver and Shaun Quegan, xe2x80x9cUnderstanding Synthetic Aperture Radar Images,xe2x80x9d Artech House, Boston, 1998, and Walter G. Carrara, Ron S. Goodman and Ronald M. Majewski, xe2x80x9cSpotlight Synthetic Aperture Radar Signal Processing Algorithmsxe2x80x9d Artech House, Boston, 1995.
The conventional GMTI implementations described above require multiple receive antennas with different phase centers, from which multiple channels of SAR signals are collected, and multiple SAR images are constructed. Clutter rejection is obtained by taking the difference of the SAR images. What is needed is a system that can overcome the problems associated with channel-to-channel mismatch of the conventional GMTI radar system, and thereby simplify the radar calibration requirements. What is also needed is a system that is lower in cost, size, weight and power than the conventional GMTI radar system.
The present invention is a system and method for radar detection of moving targets. The radar system of the present invention has only a single transmit phase center and a single receive phase center. In other words, only a single channel of received data is utilized. The present invention thus eliminates the problems associated with channel-to-channel to mismatch, thereby significantly reducing the cost of the GMTI radar system and making the radar system substantially easier to calibrate. Additionally, the reductions in size, weight and power achieved with the present invention can be significant.
The method of the present invention includes radiating a set of pulses to illuminate a desired scene. The pulses are reflected off of a target and a set of return signals are received. The pulse returns are then digitized, resulting in a set of data returns. Motion compensation is then performed on the entire set of data returns. A plurality of blocks of the motion compensated data are selected that are highly overlapping in time. A plurality of images is then generated from the plurality of blocks of motion compensated data. A consecutive set of the plurality of images is then selected. Lastly, moving objects represented within the consecutive set of images are identified.
The moving objects are identified by first extracting a time series of reflection coefficients for at least some of the pixel locations of the consecutive set of images, producing a pixel function for each pixel, thereby representing the consecutive set of images by a collection of pixel functions. Next, the linear phase component from each pixel function is removed, producing phase-compensated pixel functions. Next, a high pass filter is applied to each phase-compensated pixel function, and the output of the filtered pixel function is integrated to produce a corresponding pixel value. Finally, possible moving objects are selected by identifying pixels with pixel values greater than a pre-determined threshold.